The increased surface to volume ratio afforded by nanofibers has significant influences on a broad range of applications. In particular, in filter performance, which is based on producing the highest flow rate while trapping and retaining the finest particles without blocking the filter, nanofibers have improved interception and inertial impaction efficiencies.
In practice, the nanofiber medium on itself is soft and fragile and cannot be used alone as air filters. Nanofibers currently can only be coated on a rigid substrate to form a composite that can be handled readily. Most often the substrate is a non-woven microfiber medium. Nanofiber coating can be either produced by electrospinning method or melt-blown process, with diameter of electrospun fibers usually smaller than that of melt-blown fibers. Electrospun nanofibers are typically produced at such low rates as to be excessive in cost for many applications, and melt blown nanofibers are relatively expensive when compared to standard filter media. Even islands-in-the-sea nanofibers, which can be produced at high rates, are costly to produce because they require a removable sea and a process step to remove the sea. Melt blown nanofiber processes or melt film fibrillation process that randomly lay down fibers do not provide adequate uniformity at sufficiently high throughputs for most end use applications.
Nonwoven webs have been disclosed for use in air filtration media. U.S. Patent Application 2006/0137317 and U.S. Pat. No. 8,282,712 disclose a filtration medium consisting of a 2-layer scrim-nanofiber (SN) structure for air filters. The SN structure has at least one nanofiber layer of fibers having diameters less than 1 μm and at least one upstream scrim layer. The medium has sufficient holding capacity for dust particles that efficiency loss and pressure loss across the medium are minimized during use. The medium has good flux/barrier properties (i.e., high efficiency and low pressure drop). However, the dust-loading capacity is lower than the desired value in certain industrial HVAC applications when filters are challenged with very small dust particles, which can occur when the HVAC system is designed and constructed to have lower efficiency pre-filters in front of the high-efficiency final filters. In the SN structure, the scrim is typically made of nonwoven webs of fibers with fiber diameters of 14 to 30 microns which can pre-filter out particles larger than about 5 microns in size. The remaining particles will reach the thin nanofiber layer and quickly fill up the pores and plug up the filters. As a result, filter resistance increases rapidly and thus shortens filter life. Attempts have been made to increase the dust-loading capacity by increasing the basis weight and thickness of the scrim layer.
U.S. Pat. No. 6,521,321 discloses attempts to increase life-time of air filters by layering at least 6 to 7 coarse and fine fiber webs alternatively in a gradient-structure media (e.g., SNSNSN). The number of layering required makes this approach economically unattractive.
U.S. Pat. No. 7,125,434 discloses attempts to use a deep gradient-density filter consisting of three zones of materials for filtering biopharmaceutical fluids. The filter has a depth of at least 1.27 cm and is designed for liquid filtration. The thickness is prohibitive for pleated air filtration uses.
On the other hand, electrically-charged nonwoven webs are commonly used as filters in respirators to protect the wearer from inhaling airborne contaminants. The electric charge enhances the ability of the nonwoven web to capture particles that are suspended in a fluid. The nonwoven web captures the particles as the fluid passes through the web. Electrically-charged dielectric articles are often referred to as “electrets”, and a variety of techniques have been developed over the years for producing these products. Electrostatically treated meltblown filter media, as described in U.S. Pat. Nos. 4,874,659 and 4,178,157, perform well initially, but quickly lose filtration efficiency in use due to dust loading as the media begin to capture particles and the electrostatic charge thus becomes insulated. In addition, as the effective capture of particulates is based on the electrical charge, the performance of such filters is greatly influenced by air humidity, causing charge dissipation. Fibrous electret webs have been produced by electrizing the fibers or the fiber webs, deliberately post-charging them with a corona discharge device (U.S. Pat. Nos. 4,588,537, 6,365,088 and 6,969,484), tribocharging which occurs when high-velocity uncharged jets of gases or liquids are passed over the surface of a dielectric film (U.S. Pat. No. 5,280,406) or adding certain additives to the web to improve the performance of electrets.
U.S. Pat. No. 8,277,711 disclosed a nozzle-less centrifugal melt spin process. The resulting nanofibers were laid on a belt collector to form web media using the process of WO 2013/096672. This technology makes it possible for mass production of nanofibrous web with much higher throughput and much lower manufacturing cost.
What is needed is an improved electret nanofibrous web with higher air filtration efficiency and lower air resistance.